This invention relates to an ignition device for a gas discharge lamp.
Gas discharge lamps are usually driven by a source of alternating electrical current and are controlled by means of a regulating device such as an inductive choke or an electronic "ballast" circuit, which controls the lamp voltage and current, together with an ignition device to provide high voltage pulses which initiate the discharge arc in the lamp. The ignition device must only operate until the lamp ignites and must then be disabled. Such ignition devices can be controlled by manual switches, timers or voltage detection circuits.
The ignition pulse voltage which is required to ignite the lamp may be in the region of 20 to 65 kV. Because of the large step up ratios required to obtain such voltages from a standard AC mains voltage, step-up transformers which are used in typical igniters tend to be bulky and costly. To reduce the cost of such transformers, they are usually short-time rated to allow small frame sizes to be utilised, and multiple stages may be used to overcome insulation breakdown problems which may otherwise occur due to the high voltages involved. Due to the higher voltages necessary for re-ignition of a hot lamp (say, three or four times the cold ignition voltages), and the difficulty of achieving these voltages from normal mains supplies, hot lamp re-ignition is often not provided for commercially available ignition devices.
Certain ignition devices incorporate spark-gap oscillators which operate at high voltages and which are sensitive to changes in humidity and atmospheric pressure. Such devices may wear or need adjustment, which is a disadvantage.
In order for ignition to occur reliably, the lamp ignition pulse should be applied at or near the peak of the electrical waveform applied to the lamp. The ionisation caused by the ignition pulse dissipates rapidly in high pressure lamps such as high pressure sodium or metal halide lamps, so that timing is more critical with such lamps than with relatively low pressure lamps such as low pressure sodium or fluorescent lamps. The timing of the pulse is even more critical when the lamp is at full working temperature and must be restarted, since its internal pressure will then be at its highest level.
Known lamp ignition circuits produce a rapid sequence of high power, high frequency pulses in the hope that one of the pulses will be applied to the lamp close enough to he waveform peak of the lamp electrical supply to cause ignition. Such a system obviously has a relatively high power consumption, since many more pulses are provided than are required to start the lamp. Due to the large number of applied pulses and the power contained in them, the radio frequency interference (RFI) generated is considerable.